This invention relates generally to photo-sensitive circuits, and more particularly to photo sensitive circuits used in pixel cells.
Most commercial complementary metal oxide semiconductor (CMOS) photo-sensors have a roughly linear response to incident light until the sensor reaches saturation, after which time any further conversion of light photons into electric charge exceeds the storage capacity of the photo-sensor. The linear range of CMOS photo-sensors commonly extends over about three orders of magnitude, or about 60 dB. Such a response range is suitable for high, xe2x80x9cflatxe2x80x9d illumination conditions such as video conferencing, video recording, or some conventional, still-camera imaging applications. However, in some applications, the linear response provided by many CMOS photo-sensors is not the most desirable.
For example, a linear response is usually undesirable in applications requiring a high dynamic range. The term xe2x80x9chigh dynamic rangexe2x80x9d refers to conditions under which there is a large variation of light intensity within a single scene. Bright, sunny days are one example of a high dynamic range condition under which the linear output of many CMOS photo-sensors is less than ideal, because objects cast deep shadows, and it is difficult for conventional CMOS photo-sensors to provide the desired level of detail in both the shadowed areas and in the brightly lit areas. Other situations in which the linear output of conventional CMOS photo-sensors may not be desirable include low light machine imaging applications, surveillance, imaging of driving scenes at night, and similar situations.
In addition to having a limited dynamic range, conventional CMOS photo-sensors do not provide blooming protection for cases in which the photo-sensor continues to generate charge even after the photo-sensor is xe2x80x9cfull,xe2x80x9d and no additional charge can be stored. In photo-sensors without blooming protection, excess charge generated by a photo-sensor xe2x80x9cblooms,xe2x80x9d or leaks to nearby circuitry, including other image sensors/pixel cells, and causes degradation of the image output.
In order to achieve a wider response to incident light, some pixel cells have been constructed to extend the dynamic range of a pixel cell by biasing a transistor connected to the output of a photodiode in the subthreshold region using a fixed control voltage. By biasing the output transistor in the sub threshold region, a close approximation of a logarithmic response to incident light can be achieved. This approach can extend the dynamic range of the pixel cell, but since the response of the pixel cells is dependent on the barrier characteristics of the transistor used, and in addition the barrier characteristics can vary from cell to cell, such a method can introduce image noise.
Another method of increasing the dynamic range of a pixel cell includes applying a varying control voltage to the transistor, instead of biasing the transistor in the subthreshold region using a fixed voltage. While this is an improvement on the subthreshold bias method, without some sort of isolation of the photo-diode, the capacitance of the photo-diode has an adverse impact on the charge-to-voltage conversion of the pixel cell. In order to provide photo-diode isolation, an additional transistor is interposed between the xe2x80x9crange extensionxe2x80x9d transistor and the photo-diode.
Refer now to prior art FIG. 1, which will be referenced in discussing the xe2x80x9cisolationxe2x80x9d transistor. Transistor 104 is the xe2x80x9crange extensionxe2x80x9d transistor, transistor 102 is the xe2x80x9cisolationxe2x80x9d transistor, and transistor 106 is used to drive the output of the pixel cell, which is represented by the charge stored in floating node 108. By biasing transistor 102 slightly xe2x80x9con,xe2x80x9d the capacitance of photodiode 103 can be isolated from storage node 108 during the time when charge is being generated by photo-diode 103 and stored in floating node 108. A time-varying signal is applied to the control gate of transistor 104 to achieve the desired extension of the pixel cell""s dynamic range. During readout of the pixel cell, transistor 102 is turned off, so that the voltage at floating node 108 can be properly determined.
However, when transistor 102 is in the off state, photodiode 103 continues to generate current in response to light. If the intensity of the light is sufficient to cause photodiode 103 to generate more charge than photodiode can store, then photodiode 103 will bloom, causing charge to be distributed to nearby circuit components. This occurs because transistor 102, when in the off state, prevents charge from leaving photodiode 103 after being generated. As a result, a circuit such as that illustrated in prior art FIG. 1 provides no blooming protection during a readout period of the pixel cell. What is needed, therefore, is a pixel cell that can provide both an extended range dynamic response and blooming protection.